Bleaching process



y 1934- J. CAMPBELL ET AL v 1,957,938

BLEACHING PROCESS Filed Dec. 9, 1931 INVENTOR- WIW Patented May 8, 1934BLEAOHING PROCESS John Campbell and Lancelot 0. Bolleaton, Glens Falls,N. Y., assignors to International Paper Company, New York, N. Y., acorporation of New York Application December 9,, 1931, Serial No.579,896

This invention relates to an improved method of bleaching fibrouscellulose materials such as paper pulp, wood pulp, straw, bagasse,esparto grass, cane fiber, and similar materials. More particularly ithas reference to a process of the type disclosed in our pendingapplication Serial No. 487,758 filed October 10, 1930, in which thematerial to be bleached is subjected to the action of a gaseousbleaching agent, such as free chlorine gas, under suitable conditions atone or more stages of the process.

It is contemplated that the material shall first be placed in acondition in which it is made available for thorough impregnation withthe gaseous agent. This involvesreducing the ma--terialtoacertainstateofdrynesswhileatthe same time maintaining thefibers or the like in a relatively loose condition so that air may bepresent within the individual cells of the material and on their outersurfaces in the spaces or interstices between the'cells. The water inthe material which may suitably make up somewhere between 15% and 85% ofthe entire composition of the pulp, and preferably between and 75%,should simply be in the form of a thin film lining the inner and outersurfaces of the hollow tubes or envelopes constituting the cells of thematerial leaving room both within and without for air and, at certainstages in the process, for the gaseous bleaching agent.

Fibrous cellulose may conveniently be brought into this state in avariety of different ways. For example pulp taken from the roll of anordinary wet machine and allowed to expand will be found to carry anappropriate amount of moisture distributed in the manner indicated. Soalso the material may be brought to the desired state by passing acomparatively low density solution, containing, say, 4 parts of thesolid substance and the balance water in 100 parts of solution, into acentrifugal machine of suitable construction and whirling the materialuntil it is reduced to the proper state of dryness. In the expulsion ofa large part of the water by centrifugal .action in this way it will befound that the remaining water will simply,line the inner and outersurfaces of the cells and air will enter the material to fill up thevoids. Still another method of producing pulp in the desired state is tosqueeze a relatively wet pulp in a suitable screw press by forcing itthrough a perforated plate by the action of a screw. In this way adesired portion of the water may be squeezed from the material and whenit is allowed to expand after compression it will take in acorresponding amount of air to fill the spaces within and between thecells. The water remaining inthe material when produced in any of theforegoing ways willbe inthe form ofathinfilm of finely dividedparticles.

The bringing of the material into the definite state pointed out aboveis an important step in the process as a whole since the reaction withthe gaseous agent can not otherwise be effectively and efiicientlycarried out. It will be found that those skilled in the art will becapable of so controlling the operation of a wet machine, a centrifugalmachine, or a screw press or any other machine adapted for the samepurpose as to produce the desired result without undue attention.

Material which has been prepared in any of these ways will be wellsuited to a bleaching operation, involving the introduction of a gaseousagent into a treating zone maintained under a partial vacuum, as will beexplained more fully hereinafter. A more rapid and more effective bleachmay be brought about, however, if the material in the general conditionmentioned contains, prior to the bleaching operation, certain chemicalsubstances which will react with the gaseous bleaching agent to increasethe activity of the agent. For example where free, gaseous chlorine isemployed as the bleaching agent it will be found that the prior additionof certain alkaline substances and various salts of such substances willgreatly increase the activity of the chlorine. Among the substancesparticularly suited for this purpose may be mentioned sodium, potassium,calcium or magnesium oxides,

or suspension of the fibrous material before it is fed to thecentrifugal machine, screw press, or the like, for the removal of theexcess water. In the fibrous material which is ready for the bleachingaction the chemical substances may be in solution or suspension in theparticles of water which line the inner and outer surfaces of the cellsor a part or all of these substances may be chemically combined with thefibrous material itself. It is probable that these substances willcombine with the free chlorine, later introduced, to form suitablehypochlorites which contribute their bleaching effect.

Having brought the pulp or similar material into the particular statespecified, either with or without the addition of chemical substances ofthe nature mentioned, the bleaching reaction with a gaseous agent may becarried out in the way set forth in our pending application hereinbeforereferred to. For example the relatively dry material may be introducedinto a suitable vessel which may then be closed and made gastight. Avacuum pump may be connected with the interior of the vessel to remove alarge percentage of the air including that previously held within thecells and in the interstices between the cells. A vacuum of any suitabledegree, preferably around 25 inches of mercury or even greater mayadvantageously be produced within the vessel. Simultaneously with theremoval of air from the vessel at one point free chlorine gas may beintroduced at another point or if desired the introduction of the gasmay be made subsequent to the removal of the desired portion of the air.At first the chlorine will react with the chemical substances added tothe pulp, and with certain constituents of the pulp which impart anobjectionable color to it. As a result no appreciable increase inpressure takes place. When this reactionhas been carried to a certainpoint, however, pressure is gradually built up within the vessel untilsubstantially atmospheric pressure is attained. The bleaching reactionis ,al-

- most instantaneous since the gas is brought very quickly into quiteintimate contact with all portions of the material. It is not necessaryto leave the chlorine in contact with the material for any appreciabletime. As soon as the desired pressure has been built up within thevessel, which may be just atmospheric or slightly above or below,depending upon the difllculty with which the reaction takes place andupori the harmful reactivity of the material to the gas, the excesschlorine may be removed and again replaced by air.

A simple way in which this may be efi'ected is to blow air underpressure, of, say, 15 or 20 lbs. through the material. This will, in alarge measure, drive out the free gas and replace it with air. In orderto avoid a wastage of the free chlorine and also to avoid a discharge ofthis gas into the atmosphere with resulting hardship upon the operativeswe have found it desirable to pass the mixture of air and gas, leavingthe vessel containing treated pulp, into a second vessel containing pulpto be treated. The untreated or partially treated pulp serves as afiltering agent for the unused chlorine since this will immediatelyreact with the pulp or the chemical substances with which it may beimpregnated. The air may simply be permitted to blow through this secondvessel into the atmosphere and it will be found to be in a quite purestate upon discharge. In lieu of forcing the excess gas by means of airunder pressure, as explained, it may be drawn from the first vessel intothe second by opening up a communication between the two and connectingthe second chamber, containing the fresh material to be bleached, withthe vacuum pump.

After a batch of material has been bleached to a desired degree in themanner indicated it is ness of the original batch by the appropriate useof a centrifugal machine, wet machine, screwpress, or the like. Thepartially bleached and washed material, preferably impregnated with anew quantity of alkaline substances, may then be given a secondbleaching action by the use of vacuum and the introduction of chlorineor other gaseous bleaching agent in the same manner as before. Afterthis second bleaching action the material may again be washed and, ifdesired, treated in the same way as before to carry on a third bleachingreaction under substantially the same conditions. It has been found thata much more thorough bleach may be effected without deteriorating thefibrous structure of the material by carrying out the process in severalstages. A two stage treatment, each stage being preferably ofsubstantially the same intensity, will be found to work very goodresults with most forms of pulp. In dealing with some forms of material,however, such as kraft pulp which it is desired to bleach entirelywhite, it may be found that a three stage treatment, all stages beingpreferably substantially the same will produce the most favorableresults. A greater number of similar treatments may be employed shouldit be found necessary to effect a more gradual bleach upon each stage orshould some difliculty be encountered in attempting to fully bleach thematerial in fewer stages of treatment.

Following the final bleaching reaction the material is subjected to afurther washing to remove impurities and to a treatment with causticsoda or similar strong alkali for the purpose of removing all traces ofchlorinated products. Certain of the converted products resulting fromthe chlorination of the coloring matters will not dissolve in water andcan not be effectively removed without special treatment. A solution ofcaustic soda will be found particularly effective for this purpose.Finally the caustic soda and converted substances should be removed fromthe pulp by a thorough washing. The pulp is then ready for furtherdisposition of any nature as for the production of paper or othersimilar products.

For the purpose of more clearly illustrating the preferred sequence ofoperations a diagrammatic view has been prepared in which the variouspieces of equipment that may be advantageously employed in the processare shown schematically. Referring now to this drawing pulp in the formof slush may be received from the mill through apipe 10anddumpedintoamixbox ilofany suitable construction. If alkaline substancesof the nature already specified are to be added to the pulp prior tochlorination they may be introduced through suitable connections from atank 12 containing a strong solution of the alkali. A caustic sodasolution has been found particuuarly emcacious for the purpose. It hasbeen found that an appropriate mixture in the mix box may consist offrom 2 to 5 parts of caustic soda in 1000 to 3500 parts of watercontaining 100 parts of the dry pulp. If other alkaline substances aresubstituted for the caustic soda they should preferably be in suchproportions as to produce substantially the same degree of alkalinity.It should be understood, however, that larger or smaller quantities ofthe alkali than specified may be added and that, if desired, the alkalimay be omitted altogether.

The relatively low density mixture present in the mix box 11 may bedischarged into the hopper 13 of any suitable form of water extractingequipment. This hoppermay, for example, be

associated with the chamber of a screw press 14 which is adapted tosqueeze the solid material through a series of perforations 15 at theforward end of the press. The liquor which is squeezed from the pulp maybe withdrawn through a pipe 16 communicating with the bottom of thescrew press chamber and may dispose of this liquor in any suitable way.It is preferably re-used at some point in the process where its chemicalcontent may be used to advantage or it may be simply concentrated byevaporation of a large part of the water and then. passed to the tank12. The small lumps or nodules forced through the open ings 15 of thepress may be discharged into any suitable form of open container 1'7.Here the pulp fibers will be permitted to expand with the result thatair will be introduced into the cells and into the interstices betweenthe cells.

From the bottom of the hopper or storage vessel 17 the pulp may be ledby suitable chutes 18 and 19 into any one of a plurality of chlorinationchambers. Two chambers or vessels 20 and 21, are shown for this purpose.Only one chamber is required for the treatment of a particular batch butthe provision of two or more is considered advantageous inasmuch as .itpermits a more or less continuous operation. Assuming that a quantity ofthe material is discharged through the chute 18, by a suitable trapdoorcontrol, into the chamber 20, the latter is thenmade gas-tight by theapplication of a cover 22, which may propriately clamped down for thiswe chambers 20 and 21 are preferably mounted for rotation abouthorizontal axes so that at a later stage in the operation, the materialmaybe dumped through the opening at the top after removal of the cover22. For this purpose the containers are preferably provided withtrunnions at abouttheir midpoints adapted to rotate within bearingsprovided in supports 23.

Having filled the vessel 20 and made it gastight by the application ofthe cover 22, the air is exhausted from the interior of the vessel bymeans of a suitable vacuum pump 24. This pumpmay be connected with theinterior of the vessel by a line 25 terminating in a pipe extendingthrough one of the supporting trunnions. As shown, the construction mayconveniently be such that the same line 25 is connected with a pipeextending between the interiors of the two chambers 20 and 21. Suitablevalves 26 may then be provided to place the appropriate one of thechambers under the control of the vacuum pump. After the air has beenexhausted to the desired degree, so that a vacuum of, say, 25 inches ofmercury, more or less, is created within the vessel 20, free chlorinegas may be admitted as, for example, through the opposite trunnion ofthe vessel, or the chlorine may be admitted simultaneously with thewithdrawal of the air. For this purpose a tank 27 containing chlorine inliquid form under pressure may be connected by lines 28 to pipes 29passing through these opposite trunnions. By proper adjustment of thevalves 281 in the lines 28 chlorine may be admitted to the vessel untilthe desired pressure. preferably atmospheric, is estab lished within thevessel. Almost immediately thereafter the free chlorine which has notreacted 'with the pulp or-chemicals added to the pulp may be forced outof the vessel by air introduced under pressure. For this purpose ablower 30 may be provided, this.being connected by a line 31 with thesame pipe 29 which islconnected with the chlorine tank. The air under asuitable pressure,

of say 20 pounds, sweeps throught-he material tight, the material issubjected to a vacuum by 150 within the vessel 20 and dilutes andremoves a large portion of the remaining free chlorine. The air with itschlorine content is preferably passed from the vessel 20 into the vessel21, this being permitted by opening of both of the valves 26 and closingof a valve 32 in the line 25. The mixture of air and gas in the vessel21 will be subjected to a filtering action by contact with the freshcharge of pulp previously introduced into the vessel. The air may bepermitted to discharge into the atmosphere through a valve controlledoutlet 33 or, if desired, the cover 22 for this vessel may simply beleft partially or completely open during this stage of the operat;on.'Subsequently the material in the vessel 21 will be subjected to the sametreatment previously. given the material in vessel 20 and the remainingunspent chlorine in vessel 21 will be blown through a new batch ofmaterial in vessel 20. By the alternate use of a plurality of vessels inthis way the operation may be made substantially continuous. I

The material in the vessel 20, which has now been partially bleached, isdumped into a hopper or storage chamber 34. This may be effected in theconstruction shown by simply removing the cover 22 and rotating thevessel upon its supporting trunnions, the pipe connections 25 and 29being suitably attached and packed to permit this rotation. Any suitablemeans, such as a conveyor screw 35, may be employed to feed thematerial, which is still in a substantially dry state, into an inclinedchute or passageway 36 which discharges at its lower end into a storagetank 37. A suitable spray device 38 may be provided to discharge arelatively large quantity of water into the pulp as it descends thechute 36 or as it reaches the tank 37. Sufficient water may be added inthis way to restore the pulp to its original low density condition inwhich it carries from 3 to 10% fibrous material and 90 to 97% water. Ifdesired, the spray- 38 may be supplied with a suitable aqueous solutionof caustic soda or other alkaline substances employed to react with thechlorine. The pulpmay be subjected, if desired, to a preliminary washingoperation before the dilution to the state mentioned and before theaddition of alkaline substances which constitutes preparation of thematerial for a subsequent bleaching operation. The relatively lowdensitypulp mixture in the tank 37 may be forced by a pump 39 through aline 40 into a second mix box 41. Here a quantity of caustic soda orother alkaline agent may be added from a tank 42 to bring the alkalinityof the pulp to substantially the same state as is maintained in the mixbox 11. If sufficient alkali has been added through the spray 38 nonewill be required from the tank 42. While it is deemed preferable toobtain the same degree of alkalinity in the box 41 itwill be understoodthat this may suitably be made either weaker or stronger or the alkalimay be omitted altogether at'this point. I n

The pulp mixture from the mix box 41 may be subjected to substantiallythe same treatment as the original pulp taken from the mix box 11. Itmay be first passed 'to a water extracting machine, such as a screwpress 43, which discharges the pulp in a relatively dry state,containing between 20 and 85% of water, into a hopper or storage vessel44. From this hopper the material may be alternately introduced into oneor another of a pair of chlorination chambers 45 and 46. Assuming thatit is passed to the chamber 45 and the cover is applied to this chamberto render it gasthe action of a second vacuum pump 47, or, if desired,by a direct connection with the vacuum pump 24. Upon expulsion of theair to a desired degree from the'material and the interior of thevessel, chlorine may be introduced from a tank 48 through one of thelines 49. When sufiicient chlorine has been introduced to restore thepressure to substantially atmospheric, or any other desired pressure,the excess chlorine may be removed by air under pressure derived fromthe blower 30 or from a separate blower 50, as shown. Valves 51 and 52will be suitably adjusted to direct the air-from the blower through oneof the supporting trunnions of the vessel 45, assuming this is the onein which the chlorine treatment has just been completed. This air,together with the chlorine swept from the cells and interstices of thefibrous material, may then'be passed into the other vessel 46 by properadjustment of valves 53 and 54. Here a new charge of material will serveto remove the excess chlorine. so that the air discharged into theatmosphere through a valve-controlled outlet 55 will be quite pure.

After the second bleaching reaction has been completed, the vessel 45may be rotated upon its trunnions and the material may be dumped throughthe opening at the top of the vessel, upon removal of the cover, anddischarged into a hopper or storage tank 56. From this tank it may befed by a screw 57 to an inclined chute 58 which communicates at itslower end with a tank 59. A spray 60 adapted to direct a supply of washwater into the pulp may be located either adjacent the chute 58 or thetank 59. In the latter the pulp is maintained in a relatively lowdensity solution and may be adapted to overflow to an inclined chute 61which discharges the material into a mixing tank 62. Acaustie sodasolution of suitable strength maybe introduced with the water throughthe spray 60 or it may be separately introduced through a spray 63located adjacentthe chute 61. Inthe mixing tank the material will bethoroughly mixed by being forced tofollow a tortuous path around theends of the bafiies shown. The chlorinated products which are notdirectly soluble in water will be converted by the caustic soda intowater-soluble compounds and may subsequently be removed from the pulp byan appropriate washing treatment. The caustic soda treatment may bepreceded, replaced, or supplemented by a wash with a solution.containing sulfur dioxide. For example suli'ur dioxide may be containedin the water introduced through either the spray 60 or 63. The materialmay be discharged through a pipe 64 and passed to any suitable point forfurther purification and manufacture into paper or other final products.

While a two-stage bleaching treatment has been illustrated in thedrawing, it will 'be understood that three or more stages may beemployed, if desired. On the other hand, if the material is relativelywhite at the outset, or is not to be bleached to a pure white color, itmay be found that a single stage of treatment will be sufllcient.Ordinarily,'however. it will be found that a multistage treatment inwhich each stage is adapted to contribute its proportionate share to thefinal bleaching will produce the most satisfactory results. Theproduction of a pure white pulp from a relatively dark, fibrous mass,resulting for example from the Kraftprocess, may be produced withoutdestruction of the fibers by appropriate control of a multi-stageprocess. Furthermore, an economy in the use of the bleaching agent andthe reacting alkaline chemicals may beeffected through the use of amulti-stage treatment.

While certain definite operating conditions and methods, as well ascertain specific forms of apparatus, have been particularly pointed outit will be understood that various modifications may be made withoutdeparting from the general principles and scope of the invention. Theterms and expressions employed herein are to be considered as terms ofdescription and not of limitation.

What we claim is:

1. A method of bleaching fibrous cellulose material which ,compisessuccessively subjecting it to a reaction with free chlorine gas inaplurality of stages, removing air from within and between the cells ofthe material prior to each stage of the reaction and maintaining thematerial in a substantially dry state during each stage of the chlorinereaction.

2. A method'of bleaching fibrous cellulose material which comprises.successively subjecting it to a reaction with free chlorine gas in aplurality of stages, removing air from withinand between the cells ofthe material prior to each stage of the reaction, maintaining thematerial in a substantially dry state during each stage of the chlorinereaction, and reducing the material to a fluid, aqueous suspensionbetween the successive stages.

3. A method of bleaching fibrous cellulose material which comprisesreducing a liquid suspension of the material to a substantially drystate but with the fibers relatively free to permit the retenti" of air,withdrawing air from the-materialfi's'ubjecting the material to reactionwith a gaseous bleaching agent, adding water to the bleached material,again reducing the material to a substantially dry state and ithdrawingair therefrom, and again subjecting he material to reaction with" agaseous bleaching agent.

4. A method of bleaching fibrous cellulose material which comprisesadding caustic soda to an aqueous suspension of the material, reducingthe material to a substantially dry state with air between the-fibers,withdrawing air from the material, subi'ecting the material to reactionwith a gaseous bleaching agent, and washing the material with an aqueoussolution of caustic soda.

- 5. A method of bleaching fibrous cellulose material which comprisesadding caustic soda to an aqueous suspension of the material, reducingthe material to a substantially dry state with air between the fibers,withdrawing air from the material, subjecting the material to reactionwith a gaseous bleaching agent, creating an aqueous suspension of thebleached material and repeating the same sequence of steps.

6. A method of bleaching fibrous cellulose material which comprisesadding an alkaline agent to an aqueous suspension of thematerial,-reducing the material to asubstantially dry state with airbetween the fibers, removing air from the material with an aqueoussolution of an alkaline agent.

terial, subjecting the material to reaction with a gaseous bleachingagent, and washing the material which comprises successively subjectingit to a reaction with free chlorine gas in a plurality of stages, thematerial being maintained in a substantially dry state during each stageof the chlorine reaction and being subjected to a vacuum to remove airfrom within and between the fibers prior to reaction with chlorine ineach stage.

9. A method of bleaching fibrous cellulose material which comprisesremoving most of the water from an aqueous suspension of the material toreduce it to a substantially dry state, removing air from within andbetween the fibers of said material, introducing a free gaseousbleaching agent, blowing air through the bleached material to remove theexcess gaseous agent from between the fibers, and washing the materialto remove impurities. I

10. A method of bleaching fibrous cellulose material which comprisesadding an alkaline agent to an aqueous suspension of the material,removing most of the water from said aqueous suspension of the materialto reduce it to a substantially dry state, removing air from between thefibers of said material, introducing a free gaseous bleaching agent,blowing air through the bleached-material to remove the excess gaseousagent from between the fibers, and washing the material to removeimpurities.

11. A method of bleaching fibrous cellulose material which comprisesadding caustic soda to an aqueous suspension of the material, removingwater from the material and allowing it to expand to absorb air,removing the air by subjecting the material to a vacuum, introducinggaseous chlorine into and between the cells of the material to bleachthe same, and subsequently treating the material with a caustic sodasolution.

12. A method of bleaching fibrous cellulose material which comprisesadding caustic soda to an aqueous suspension of the material, removingwater from the material and allowing it to expand to absorb air,removing the air by. subjecting the material to a vacuum, introducinggaseous chlorine into and between the cells of the material to bleachthe same, again forming an aqueous suspension of the material, andrepeating the same series of steps.

13. A method of bleaching fibrous cellulose material which comprisesadding caustic soda to an aqueous suspension 'of the material, removingwater from the material and allowing it to expand to absorb air,removing the air by subjecting the material to a vacuum, introducinggaseous chlorine into and between the cellsot the material to bleach thesame, again forming an aqueous suspension of the material, and repeatingthe same series of steps, and subsequently treating the material with acaustic soda solution.

JOHN CAMPBELL. LANCELOT 0. ROILESTON.

Ill

